The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference.
In recent years intensive studies have been made on artificial materials called biomaterials to be introduced in the human body for repairing damages therein. The body conditions offer a severe environment for these materials. The combination of increased temperatures, salt solutions, destructive enzymes, organic acids capable of forming different complexes, proteins and dissolved oxygen in the body provides a most corrosive environment. The body is also extremely sensitive to foreign materials and easily shows signs of poisoning, rejecting reactions and allergic responses.
Only a very limited number of materials is accepted in soft or hard tissue as a substrate. These materials can e.g. be used as artificial implants supporting crowns and fixed bridges in dentistry, and in maintenance and augmentation of alveolar ridges (1). They may also be used as fillings in bone defects and in periodontal pockets, as capping materials in endodontics, and in orthopaedic, plastic, ear, nose and throat surgery (2). The materials can be used as granules and bulk materials to fill bone cavities and defects, and as coatings and bulk materials for artificial joints. The oral implants are in continuous contact with both hard and soft tissues, and the implant material should therefore develop an intime contact with both hard and soft tissue.
Biomaterials are defined as non-living materials that are used in the human body, and which are intended to interact with different biological systems. These materials can be either inert, resorbable or bioactive (1).
Inert biomaterials, e.g. carbon, some ceramics, metals, alloys and certain polymers, do not cause any measurable reaction in the body. The carbons include, for example, pyrolytic carbon, glassy carbon, carbon fibers and composites and they are used as heart valve stents and in orthopaedic surgery (1). Examples of inert ceramics are Al.sub.2 O.sub.3 and ZrO.sub.2. Metals and alloys used as biomaterials are e.g. stainless steel, titanium, tantalum and certain alloys. These metals and alloys are not surface active, i.e. a chemical bond does not develop between the material and the body tissue. Their durability is difficult to control in the body, and they are mainly used in orthopaedic and maxillofacial surgery (1).
Resorbable biomaterials are typically organic polymers, e.g. PGA (polyglycolic acid) and PLA (polylactic acid) which gradually degrade in the body and disappear (1).
Bioactive materials are surface active materials able to chemically bond to body tissue. This group includes bioactive glasses, glass ceramics and ceramics. The bioactive glass is amorphous. Bioactive glass ceramics are materials having crystalline particles embedded in the amorphous glass phase. Bioactive ceramics have a crystalline structure. When the bond between the bioactive material and the body tissue is a successful one, a layer of silica rich gel is found at the surface of the glass. The bone-bonding occurs when the build-up of bone-like apatite on top of this silica gel occurs (5,7,8,9). These bioactive materials are used as bulk materials, granules and coatings.
Ceramics as biomaterials can be either inert, resorbable or bioactive (1). Bioactive ceramics are e.g. calcium phosphates and aluminium calcium phosphates and they are used in orthopaedic surgery and as dental implants. The most common problems with these materials relate to crystallization. The crystalline structure makes them difficult to work and it is troublesome to control the crystallization. The wear and degradation mechanisms as well as durability of the ceramics are not very well understood.
Bioactive glass ceramics are composites comprising crystals embedded in an amorphous glassy phase. Glass ceramics contain different crystalline phases in controlled amounts in the material. These phases are mainly controlled by heat-treatment. Ceravital.RTM. is a trademark for a glass ceramic developed in Germany and it contains a glassy phase and an apatite one. Cerabone.RTM. A-W is a trademark for glass ceramics developed in Japan. This material contains phases of apatite, wollastonite and glass (9).
Bioactive glasses have been in use for about 20 years as bone filling materials and prostheses in odontology, orthopaedy and opthalmology. Some of the existing bioactive glasses can bond to both soft and hard tissue (4, 5, 8, 9). The use of bioactive glasses is, however, restricted since they are brittle. To overcome the disadvantages due to the brittle properties, the glasses can be reinforced by making glass ceramics. Another possibility would be to use the glass as coatings on metal substrates. In this way, both the mechanical properties of the metal and the special bone-bonding property of the glass could be obtained. In prostheses prepared in this way the metal could take the mechanical load while the glass enables the prostheses to be anchored to the surrounding tissue. The thermal expansion of the glass must, however, match that of the metal, and the solubility of the glass must be low enough to provide the bond for several years (3). The existing bioactive glasses do not possess an acceptable viscosity-temperature dependence and therefore bioactive glasses described hereto are not suitable e.g. as coatings.
The bioactive glasses could, however, find a much larger field of use if glass fibre tissues, spherical granules and coated metal prostheses were available. In odontology, such glass fibre tissues could be used as reinforcements in cheek bone, and coated metal prosthesis could be used by orthopaedics to ensure a good fit in e.g. hip surgery.
Known bioactive glasses have attained a certain clinical use as bone filling materials. They tend, however, to devitrify (crystallize) and their working range is narrow. They can therefore not be used with satisfying results as e.g. coatings on metal prostheses or as glass fibre products. They cannot be manufactured using conventional methods because the curve describing their viscosity-temperature dependence is too steep for most glass forming machines. The main drawbacks relating to the existing bioactive glasses thus derive from their tendency to crystallize. Although the glasses are vitrous materials, some of them crystallize at low temperatures (about 600.degree. C.). This makes them difficult e.g. to sinter into a product or to use for the manufacturing of spherical granules. They are often also phase-separated due to their low content of silica, and the glass composition is therefore different from batch to batch. They have a narrow working range. FIG. 1 shows log .eta. as function of temperature (.eta. is expressed in dpa.multidot.s) for a bioactive glass of type 2-92 (number 39 in Table 1 below) which represents a glass with a narrow working range. The glass crystallizes as indicated by the steep part of the viscosity curve above 1000.degree. C. The narrow working range makes it impossible or extremely difficult to produce glass fibres and other fibre products, as well as to cast into various moulds. The reaction in tissue is rapid, which in some cases may cause too strong a reaction in the body. Thus the only remaining product that can be made from these glasses is granules.